1. Field of the Invention
The present invention relates to a tracking control circuit suited, for example, for use in a rotary head type digital audio tape recorder
2. Description of the Prior Art
Conventionally, as a tape recording apparatus capable of recording an audio signal with high density, a rotary head type digital audio tape recorder (hereafter a "DAT") has been employed.
In such a DAT, input audio data is recorded onto a magnetic tape wound around a rotary drum at a predetermined wrap angle using rotary heads arranged on a rotary drum. The recorded audio data on the magnetic tape may also be reproduced using the rotary heads.
To reproduce the recorded audio data on the magnetic tape correctly, the rotary heads must scan a recording track on the magnetic tape in a correct tracking state. For this reason, a tracking control method of the so-called ATF (automatic track following) system is adopted in the DAT.
As shown in FIG. 1, a magnetic tape 2 on which digital audio data is recorded in a DAT format is pulled out of a tape cassette in a DAT (not shown) and guided by inclined guides 3A and 3B. The tape 2 is wound around a rotary drum 5 at a predetermined angle interval. On the rotary drum 5, rotary heads 4A and 4B are arranged with an angle interval of 180.degree. and azimuth angles different from each other. The tape 3 is further sandwiched between a capstan and a pinch roller 7 and reeled into the tape cassette.
Recording tracks of the magnetic tape 2 are formed in an oblique direction with respect to its running direction so that they have alternately positive and negative azimuth angles. In the DAT format, an ATF recording pattern P.sub.ATF as shown in FIG. 2 is recorded at predetermined positions on the lower and upper sides of the recording tracks.
More specifically, for example, an ATF sync signal PS is first recorded at a frequency of 522.67 kHz or 784.00 kHz onto a recording track TA, which has a positive azimuth angle, along a scanning direction a of the rotary head 4A.
Subsequently, first and second ATF pilot signals P1 and P2 having a frequency of 130.67 kHz are recorded at a predetermined interval on the recording track TB1 on the precedinq side of recording track TA and on the recording track TB2 on the subsequent side of recording track TA, respectively, both of which have a negative azimuth angle, in response to the position of the ATF sync signal PS. Further, a third pilot signal P3 is recorded with a frequency of 130.67 kHz at a subsequent position on the recording track TA.
In a DAT, the first and second ATF pilot signals P1 and P2 are sampled and held based on the timing of the reproduction of the ATF sync signal PS on the recording track TA. A capstan motor 8 is driven by the difference signal, so that the rotary head 4A is operated to scan the recording track TA in the correct tracking state.
Referring back to FIG. 1, a reproduction signal S.sub.PB0, which is provided by the playback of recording tracks TA and TB on the magnetic tape 2, is amplified by a reproduction signal amplifier 9, and the resulting amplified reproduction signal S.sub.PB2 is input to a low-pass filter (LPF) 10A and an equalizer 11.
The LPF 10A passes the low frequency components of ATF pilot signals P1, P2 and P3 contained in the amplified reproduction signal S.sub.PB2, and these frequency components are supplied to an envelope detector 12 through a variable gain type amplifier (GCA) 10B. An envelope signal E.sub.ATF which consists of the basic components of the ATF pilot signals is given to a subsequent first sample/hold circuit 13A and a subtracter 14A.
The equalizer 11 performs a wave equalization of the amplified reproduction signal S.sub.PB2 depending on the ATF sync signal PS and then supplies it to a limiter 15. As a result, a signal component of the amplified reproduction signal S.sub.PB2 depending on the ATF sync signal PS is converted into a digital signal at the limiter 15 and given to an ATF sync detector 16.
In response to the detection of the ATF sync signal PS, the ATF sync detector 16 generates a first sampling pulse SP1 which rises and falls with the timing of the detection, and which is sent to the first sample/hold circuit 13A.
The ATF sync detector 16 also generates a second sampling pulse SP2 which rises and falls with a predetermined delay from the first sampling pulse SP1, and which is sent to second and third sample/hold circuits 13B and 13C.
The first sample/hold circuit 13A samples and holds the envelope signal E.sub.ATF input from the envelope detector 12 in response to the first sampling pulse SP1 and consequently sends the level of the first ATF pilot signal P1 recorded on the recording track TB1 as an addition input to the subtracter 14A.
The subtracter 14A subtracts the input envelope signal E.sub.ATF or the second ATF pilot signal P2 recorded on the recording track TB2 from the first ATF pilot signal P1 and sends a subtracted result to the second sample/hold circuit 13B.
The second sample/hold circuit 13B samples and holds the difference of the first and second ATF pilot signals P1 and P2 with the timing of the second sampling pulse SP2, and sends a resultant signal to an adder 18 of a capstan speed control system as a tracking error signal S.sub.TE.
A capstan frequency signal FG.sub.CP obtained from a capstan motor 8 in the DAT is amplified by a capstan FG amplifier 19A and input to a capstan speed control circuit 19B, which outputs a capstan error signal S.sub.CE. The signal S.sub.CE is supplied to adder 8 where it is added to the tracking error signal S.sub.TE and output to a capstan driving circuit 19C.
The capstan driving circuit 19C generates a capstan driving signal C.sub.CP depending on the tracking error signal S.sub.TE and the capstan error signal S.sub.CE and sends it to the capstan motor 8.
In this manner, the speed of the magnetic tape 2 is controlled by a level difference of the first and second ATF pilot signals P1 and P2, and the rotary head 4A is controlled so that it can scan the recording track TA in the correct tracking state by an ATF system.
The amplifying gain of the above-mentioned variable gain amplifier 10B is controlled depending on the sum of the reproduction levels of the first and second ATF pilot signals P1 and P2, and tracking control can be performed accurately by correcting changes of the reproduction levels of the rotary heads 4A and 4B.
As shown in FIG. 3, by correcting the change of the reproduction level of the rotary head 4A, for example, with a signal S.sub.WA (indicated by a broken line in FIG. 3) which is the sum of reproduction levels S.sub.P1 and S.sub.P2 of the first and second ATF pilot signals P1 and P2, the tracking error signal S.sub.TE can be precisely detected as compared with the case where the change of the reproduction levels is detected by a reproduction level S.sub.P3 of the third ATF pilot signal P3. As a result, the tracking error signal S.sub.TE can be detected with greatly increased accuracy.
The envelope signal E.sub.ATF sent from the envelope detector 12 and the first ATF pilot signal P1 are also supplied to an adder 14B, and this addition signal is fed to a differential amplifier 17A.
In the differential amplifier 17A, the difference between the addition signal and a voltage corresponding to a reference reproduction level given from a power supply source 17B is amplified and sent to the third sample/hold circuit 13C.
The second sampling pulse SP2, which is the same as that supplied to the second sample/hold circuit 13B, is also supplied to the third sample/hold circuit 13C. Thus, at the timing of the second sampling pulse SP2, the result of a comparison of the sum signal of the first and second ATF pilot signals P1 and P2 and the reference reproduction level is sampled and held, producing a head reproduction level error signal S.sub.HE. This signal is sent to the variable gain amplifier 10B, and fluctuations in reproduction level of the rotary heads 4A and 4B can thus be corrected effectively on the basis of the sum signal S.sub.WA of the reproduction levels S.sub.P1 and S.sub.P2 of the first and second ATF pilot signals P1 and P2.
However, in a DAT with such a structure, there are problems with variations due to temperature and circuit characteristics, since a tracking error is detected with analog signal processing.
In particular if such a phenomenon occurs in the first or second sample/hold circuits 12A and 12B, a drift or an offset takes place in a sample/hold characteristic with the result that correct tracking control cannot be accomplished.
Further, since the DAT contains an analog signal processing circuit, externally attached parts are indispensable, causing the entire circuit size to become large.